Controlled porosity article

ABSTRACT

In one embodiment, the present invention is a porous article including a porous portion with an average ratio between about 2:1 and about 5:1 of major pore size to minor pore size, having a substantially uniform pore distribution and a porosity of at least 70 percent, the porous article for use as a biological implant. The biological implant can be, for example, an acetabular cup.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.11/450,059, filed on Jun. 9, 2006, which is a continuation-in-part ofU.S. application Ser. No. 10/884,444, filed on Jul. 2, 2004, thedisclosures of which are hereby incorporated herein by reference.

BACKGROUND

Embodiments described relate to porous articles formed having acontrolled level of porosity that may be substantially evenlydistributed throughout portions thereof. Embodiments described hereinrelate to the forming of a porous metal by way of techniques thatminimize health hazards as well as hazards to the character of theporous metal itself.

BACKGROUND OF THE RELATED ART

Porous metal articles are used in many applications including orthopedicimplants, bone growth media, filters, sound suppression materials, fuelcells, catalyst supports, and magnetic shielding. Such porous articlesmay have open or closed porosity as well as a wide range of pore size,shape, density, and distribution. The specific structures and propertiesrequired depend on the application.

In order to provide a degree of control over the porosity exhibited bythe metal article, a solid pore former may be utilized when making themetal article. For example, a metal vapor or powder form of a metal maybe deposited a solid foam matrix to accommodate pore forming.Alternatively, the metal may be mixed with a pore former consisting ofpolymer or other suitable material beads. In either case, the poreformer is of a stable predetermined size, shape and other physicalcharacteristics. Thus, once the metal is hardened, removal of the solidpore former leaves behind an article that displays a largelypredetermined and controlled pore size and shape.

Employing a solid pore former as noted above provides a degree ofcontrol over the resulting pore size and shape within the porous metalarticle as compared to techniques which fail to make use of such a poreformer. For example, in an alternative foaming technique porosity isultimately determined by the fairly chaotic nature of a dissolved gasdispersing through a liquefied metal. In another alternative diffusionbonding or sintering technique, porosity is determined by and limited tothe naturally present space between metal powder or granules filling amold tool. In either case, the lack of a solid pore former leaves muchof the process of pore formation to chance.

In exercising a greater degree of control over porosity, techniques ofemploying solid pore formers vary. For example, as noted above, a solidfoam matrix may be employed as a pore former. This technique may includedeposition of a metal onto the solid foam matrix. This may beaccomplished by dipping the foam matrix in a slurry containing the metalin powder form. Alternatively, vapor deposition techniques may beemployed to provide a metal coating to the solid foam matrix.Regardless, subsequent evaporation and condensation of the material onthe substrate may follow to provide a porous metal article. Thereafter,the article may harden and the foam material may be removed throughvarious techniques, such as vaporization.

As also noted above, pore formers may be utilized in the form of poreforming beads mixed with a fluid or powdered metal. Once mixed, themetal may be hardened through a variety of techniques, such assintering. Subsequent removal of the pore forming beads leaves behind aporous metal structure.

While providing a degree of control to pore formation, thereunfortunately remain significant inherent limitations to techniquesemploying solid pore formers. For example, in the case of a solid foammatrix, the ability of a metal vapor or powder to penetrate and evenlycoat the matrix substrate is subject to inherent limitations relative tometal particle and matrix pore sizes. That is, with a thick enoughmatrix, the natural accumulation of metal clogged matrix pores willprevent deposition of the metal from proceeding further into the matrix.Thus, larger and thicker porous metal articles may not practically beformed by use of a foam matrix technique. In the case of pore formingbeads mixed with a fluid or powdered metal, the beads tend to be presentin somewhat of a free floating nature within the metal. As such, thebeads have a tendency to settle and fail to remain evenly distributedthroughout the mixture. As a result, the porous metal article which isformed can end up with pores that are not evenly distributed throughout.

Further, regardless of the type of solid pore former employed, itsremoval following formation of the article often subjects it tovaporization or other stressors likely to affect the physical characterof the porous metal article. This can deter the use of the porous metalarticle as an option for sensitive applications such as for use as abiological implant.

At present, selection of a technique employing a solid pore formereither limits the thickness of the porous metal article to be formed orincreases the likelihood of an uneven pore distribution throughout theporous metal article. Further, in either case, removal of the poreformer can affect the resulting physical character of the porous metalarticle.

SUMMARY OF THE INVENTION

An article of controlled porosity is provided. The porosity iscontrolled by a coated pore former that includes pore former andhomogenizing agent. The coated pore former is mixed with a material suchas a metal whereby the homogenizing agent maintains a uniformdistribution of pore former through the metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective sectional view of a blender containing anembodiment of a coated pore former for combining with a metal.

FIG. 2 is an enlarged view of an embodiment of a mixture of the coatedpore former and metal of FIG. 1.

FIG. 3 is a side cross sectional view of an embodiment of a mold toolreceiving the mixture of FIG. 2.

FIG. 4 is a side cross sectional view of an embodiment of a coldisostatic press chamber containing the mold tool of FIG. 3.

FIG. 5 is a side perspective view of an embodiment of a green article ofthe mixture of FIG. 2 recovered from the mold tool of FIG. 4 and shapedby a lathe.

FIG. 6 is an exploded perspective view of an embodiment of a porousmetal article formed from the green article of FIG. 5 and of tailoredporosity for use as a biological implant.

FIG. 7 is a flow chart describing an embodiment of forming a porousmetal article of tailored porosity for use as a biological implant.

DETAILED DESCRIPTION

Embodiments are described with reference to certain porous metalarticles of tailored porosity. These may include metal articles in theform of biological implants having a final porosity that is uniformlydistributed and exceeds 50-80% by volume of the article. However,embodiments described herein may be applicable to a host of porousarticles for a variety of uses. For example, metal filters and otherarticles described further herein may be formed by the techniquesdescribed below. Additionally, the porous articles may be of ceramicbased or other non-metal material. Additionally, articles exhibiting anydegree of porosity may similarly be formed by the techniques described.Furthermore, the tailored porosity may be in the form of a controlleduniform pore distribution or by way of multiple metal layers varyingporosity provided within a single article. Regardless, embodimentsdescribed herein allow the porosity provided within a metal article tobe both tightly controlled and tailored throughout.

Referring to FIG. 1, an embodiment of a coated pore former 100 is showncontained within a conventional v-blender 110. With additional referenceto FIG. 2, the coated pore former 100 is a pore former 250 present asparticles coated with a homogenizing agent 255 as described furtherherein. In the embodiment shown, metal 125 in powder form is mixed withthe coated pore former 100 to provide a pore forming metal mixture 200of pore formable metal. However, in other embodiments a ceramic, such aszirconia powder may be used to form a non-metal porous article.Continuing with the embodiment shown, the homogenizing agent 255 allowsthe coated pore former 100 to remain substantially evenly distributedthroughout the mixture 200 without settling. As also described furtherherein, the coated pore former 100 is an extractible particulate thatacts to physically define porosity once it has been removed. As aresult, a porous metal article 600 or portion thereof may be formed fromthe mixture 200 that displays pores evenly distributed throughout (seeFIG. 6).

Continuing with reference to FIGS. 1, 2, and the porous metal article600 of FIG. 6, the coated pore former 100 may include a pore former 250coated by a homogenizing agent 255 as noted above. The pore former 250itself may be selected based on morphology with the size and shape ofpores in the porous metal article 600 in mind. For example, pore formers250 may appear granular, bead-like, spherical, irregular or angulardepending on the pore morphology type desired in the porous metalarticle 600. In terms of size, embodiments of pore formers 250 describedherein may range roughly from about 50 to about 2,000 microns indiameter. However, a variety of other pore former 250 sizing may beemployed. Once more, a range of particle size and shape distribution ofthe pore former 250 may be employed within a given mixture 200. Forexample, a host of milling, grinding, sieving, and air classificationtechniques may be employed to provide a particular particle size and/ormorphology distribution configured. These particle characteristics maybe provided with the size and shape of the pores to be provided to theporous metal article 600 in mind.

The selection of the constituents of the mixture 200 may take intoaccount the processing conditions to be encountered. For example, a poreformer 250 may be selected that is mechanically capable of withstandingcompaction in order to maintain definition of a desired pore size andshape in the resulting porous metal article 600. As described furtherherein, embodiments of compaction may include the introduction ofpressures in the range of about 25 to about 50 ksi (thousand pounds persquare inch). Thus, the material of the pore former 250 may be selectedin light thereof. Similarly, a material may be selected for the poreformer 250 that it is compatible with fluid extraction for its removalfrom the mixture 200. Furthermore, given that sintering at elevatedtemperatures may follow extraction, pore formers 250 may be employedthat are relatively larger than the pore size to be displayed in thefinal porous metal article 600. That is, as described further herein,sintering will likely result in some shrinkage of the metal 125 therebyreducing pore size following extraction of the pore formers 250.Therefore, the pore formers 250 may be accordingly oversized relative tothe intended pore size found in the porous metal article 600.

With the above considerations in mind a variety of material types may beselected for the pore former 250. In particular, embodiments of poreformers 250 may include a host of naturally ionic materials such asmetal salts. Metal salts such as potassium chloride, sodium chloride andmixtures thereof are particularly good candidates for removal throughconventional water-based extraction techniques as described furtherherein. Additionally, metal salts are able to withstand conventionalcompaction techniques withstanding pressures of between about 25 andabout 50 ksi without any significant deformation. In one embodiment, thepore former 250 is provided in the form of an oversized potassiumchloride salt. In still other embodiments potassium sorbate or sugar maybe employed as the pore former 250.

Continuing with reference to FIGS. 1 and 2, the pore former 250 is addedto the v-blender 110 followed by the homogenizing agent 255. Thehomogenizing agent 255 may be added to the v-blender 110 through aconduit in its horizontal physical axis 175. This addition may takeplace over about a 5 to 10 minute period while mixing proceeds withinthe v-blender 110. The homogenizing agent 255 is provided in this mannerto coat the pore formers 250 as shown in FIG. 2. As such the poreformers 250 are provided with characteristics that promote their evendistribution throughout the mixture 200 once the metal 125 is providedas described below.

The homogenizing agent 255 may be a variety of materials selected tocompatibly coat the pore former 250 while also displaying a physicalreaction to the selected metal 125 of the mixture 200. For example, inone embodiment, a water soluble material such as polyethylene glycol(PEG) is used as the homogenizing agent 255 to coat the pore former 250where it is to be mixed with a metal 125 such as titanium, as describedfurther herein. In this embodiment, the PEG provides an activelydisengaging character to the surface of the pore former 250 relative tothe metal 125. Thus, the coated pore former 100 remains floating,relatively evenly distributed throughout the mixture 200. Stated anotherway, the metal 125 too remains substantially uniformly distributedrelative to the pore former 250, rather than allowing the pore former250 to settle. Alternative homogenizing agents which may be employed inthis manner include alcohols, isoparafinic solvents, and organic liquidssuch as acetone.

In an alternate embodiment, a homogenizing agent 255 may be provided tocoat the pore former 250 which provides an engaging character to thesurface thereof with respect to a given metal 125. That is, in thisembodiment, the physical reaction to the selected metal 125 is anattractive one. In this instance, each coated pore former 100 tends toadhere metal 125 thereto upon its introduction. Thus, coated poreformers 100 in this embodiment remain disbursed throughout the mixture200 in a manner that promotes closed cell porosity upon pore former 250removal, as described in greater detail below.

Continuing with reference to FIGS. 1 and 2, the coated pore former 100is provided by mixing the pore former 250 with the homogenizing agent255 in the v-blender 110 as indicated. In one embodiment, the poreformer 250 may constitute between about 70-80% of the mixture 200 as awhole, with homogenizing agent 255 making up between about 5 and 10% ofthe mixture 200. These initial components of the coated pore former 100may be mixed by conventional gentle mixing with the v-blender 110 forbetween about 10 and about 20 minutes or until the pore formers 250 aresubstantially coated with the homogenizing agent 255. This isaccomplished by adding the pore formers 250 and homogenizing agent 255to the v-blender 110 through either of the upper doors 150. Conventionalv-mixing then proceeds with the v-blender 110 rotating about itshorizontal physical axis 175.

Providing only up to the optimum amount of homogenizing agent 255 mayallow more complete compression or compaction of the mixture 200 asdescribed further herein. Therefore, while homogenizing agent 255provides a controlled pore character, in terms of pore distribution, useof only up to an amount for coating the pore formers 250 is furthered bymixing of the pore formers 250 and homogenizing agent 255 thoroughlybefore addition of the metal 125 as described.

With added reference to FIG. 6, a final porous metal article 600 mayexperience a certain degree of shrinkage and other changes viaprocessing techniques described below. Nevertheless, the ratio ofconstituents provided to the mixture 200 as described above may beconfigured with this in mind to provide a porosity of between about 60%and about 85% or greater to an overlay metal 520 (see FIG. 5).Furthermore, pores ranging in size from about 50 to about 2000micrometers may ultimately be provided to the porous metal article 600by way of techniques also described herein.

Once the coated pore former 100 is sufficiently mixed, metal 125 maythen be added to the v-blender 110 as shown in FIG. 1. With additionalreference to the porous metal article 600 of FIG. 6, the metal 125 maybe provided in the form of a powder selected in light of subsequentprocessing as well as the intended use of the porous metal article 600.For example, where the mixture 200 is to undergo compaction forsolidification, powdered metal 125 of an irregular, angular, orligamental grade or nature may be employed to provide a higher tensileor green strength following compaction. Additionally, in order topromote complete and even compaction of the metal 125 and improve greenstrength, as described further herein, the metal 125 may be provided inthe form of a metal powder having between about 1% and about 10% weightthereof in the form of a thermally decomposable conventional binder orlubricant additive.

A variety of types of metal 125 may be used to form the porous metalarticle 600. For example, in an embodiment where the porous metalarticle 600 is to be used as a biological implant, the metal 125 may betitanium, or other recognized biocompatible material. In addition totitanium, other types of metal 125 which may be employed includetantalum, cobalt chrome, niobium, stainless steel, nickel, copper,aluminum, and alloys thereof. In another embodiment the metal 125 isreplaced with a ceramic powder such as zirconia to form a non-metalporous article akin to the techniques described below.

In the embodiment shown, the above selected metal 125 or other basematerial may constitute between about 15 and about 20% of the mixture200 as a whole. Once added to the v-blender 110, mixing of the coatedpore former 100 and the metal 125 may proceed by conventional means asdescribed above. For example, in one embodiment, gentle mixing with thev-blender 110 may take place for up to about 10 minutes or until asubstantially homogeneous mixture 200 of coated pore former 100 andmetal 125 is achieved. Once this homogeneous mixture 200 is achieved,settling or separation of the coated pore former 100 is substantiallyavoided due to the presence of the homogenizing agent 255. Thus,processing of the mixture 200 may proceed without significant concernover maintaining an even distribution of pores throughout the porousmetal article 600 (see FIG. 6). The homogenizing agent 255 stabilizesthe mixture 200 allowing a more reliable distribution and precisetailoring of porosity.

Referring now to FIGS. 3-7 an embodiment of processing the homogeneousmixture 200 into a porous metal article 600 that is to be employed as abiological implant is described. However, embodiments of such ahomogeneous mixture 200 may be processed into a variety of other typesof articles for which a controlled or tailored porous metal structurewould be of benefit. Such articles may include acoustical dampeners,flow control devices, metal filters, fluid applicators, catalyticsupport structures, friction material substrates, and a host of otherdevices.

With particular reference to FIGS. 1, and 3-6 the mixture 200 istransported by conventional means from the lower door 130 of thev-blender 110 and to a mold tool 300 for consolidation or compaction. Asshown, the mold tool 300 is enclosed at its lower portion by a mandrel325 and a underlying metal 350. In the embodiment shown the underlyingmetal 350 is a compact of the same type of metal 125 as is present inthe mixture 200. As shown in FIG. 3, the underlying metal 350 is in agreen state and configured to display porosity that differs from theporosity to be provided by the mixture 200. As shown in FIG. 3, themixture 200 is poured into the mold tool 300 over the mandrel 325 andunderlying metal 350. Following compaction and other processing, themetal within the mixture 200 will make up the structure of an overlaymetal 520 of the porous metal article 600.

Continuing with reference to FIGS. 3-6, the mandrel 325 is employed toprovide a concave shape to the mixture 200 and/or the underlying metal350 immediately thereabove. However, the mandrel 325 may be of a varietyof shaping and support types depending upon the type of porous metalarticle 600 that is to be formed. In fact, the mold tool 300 may even beconfigured to provide a large block or cylinder shaped ingot for latershaping into an article of a desired final shape. In the particularembodiment shown, the mandrel 325 is provided for support of the mixture200 and the underlying metal 350 thereabove during compaction asdescribed below. Therefore, the mandrel 325 is preferably of a materialdiffering from that of the layers of mixture 200 and underlying metal350. In this manner, adherence of the layers to the mandrel 325 as aresult of the compaction process may be avoided. For example, in oneembodiment, the mixture 200 and the underlying metal 350 include metalin the form of titanium, whereas the mandrel 325 is of a solid stainlesssteel.

The embodiments shown in FIGS. 3-6 include the above-noted underlyingmetal 350. However, in alternative embodiments, any number of layers ofunderlying metal 350 may be provided (including none at all). Inembodiments employing several such layers, each may be configured todisplay its own unique pore character in the final porous metal article600. That is, each layer of underlying metal 350 may be independentlyformed to provide its own pore size, shape and distributiontherethrough. Each layer of underlying metal 350 may be formed from itsown metal-based mixture, each mixture to provide its own individual porecharacter. Further, each mixture may be followed by its own separatecompaction to a green state as described here with the mixture 200 andoverlay metal 520 shown. In this manner, multiple layers of varyingporosity may ultimately be provided to a single porous metal article 600as described further below.

In the embodiments shown, the underlying metal 350 is formed from amixture that is a titanium powder without any pore forming componentsand is thus configured to display negligible if any porosity in thefinal porous metal article 600. Thus, as described further herein, anoverlay metal 520 to display porosity and a underlying metal 350 todisplay no porosity are co-formed into the single porous metal article600 (i.e. displaying a tailored porosity therethrough).

With particular reference to FIGS. 3 and 4, the mixture 200 is pouredinto the mold tool 300 taking up the space 370 between the underlyingmetal 350, mandrel 325, and the walls 375 of the mold tool 300 itself.This space 370 is filled until the mixture 200 reaches about theneckline 380 of the mold tool 300.

Once the mold tool 300 is filled, the mixture 200 may be dried forremoval of the homogenizing agent 255 (see FIG. 2). For example, in oneembodiment, drying may take place at between about 25° C. and about 75°C. for between about five minutes, and about one hour depending on thevolume of the poured mixture 200, its exposure to outside air, and otherfactors. Alternatively, removal of the homogenizing agent 255 may bebypassed until later processing as described below.

The mold tool 300 is sealed with a plug 450 and may be rotated ortumbled several times to help eliminate any potential stratification ofthe mixture 200 within the mold tool 300. Additionally, the mold tool300 may be tapped several times to assure proper filling. For example,in one embodiment, the mold tool 300 is forcibly impacted 20 to 60 timesonto a table. The repeated landing of the mold tool 300 on the tablehelps to ensure that the mold tool 300 is completely filled without anyvoids in the mixture 200. Topping off of the mold tool with additionalmixture 200 would then eliminate any such voids.

Once the mold tool 300 has been finally sealed with the plug 450consolidating of the mixture 200 may take place by application of a ColdIsostatic Press (CIP). In the embodiment shown, the mold tool 300 ismade up of rubber or other CIP compatible material. With the mold tool300 sealed with the plug 450 it may be placed in a CIP chamber 400. TheCIP chamber 400 is filled with water or other CIP compatible medium andactivated. The CIP chamber 400 may be activated by conventional means tocompact the mixture 200 within the mold tool 300. In one embodiment themold tool 300 is subjected to between about 25 and about 50 ksi ofpressure in the CIP chamber 400. However, pressures outside of thisrange may be employed. Additionally, alternative techniques ofphysically stabilizing the mixture 200 of FIG. 2 may be employedaltogether. For example, conventional die compaction, powder extrusion,or metal injection molding techniques may also be employed.

In an alternate embodiment in which powder injection molding (PIM) isemployed, a conventional water, acetone, or alcohol soluble binder maybe mixed with the metal 125 ahead of time. In this embodiment poreformers 250 may subsequently be mixed in to form the mixture 200 withconventional PIM processing to follow for solidification.

Once the mixture 200 of FIG. 2 has been consolidated into a solid form,it may be removed from the mold tool 300 for further processing as agreen article 500. In the embodiment shown, the mixture 200 has actuallybeen compacted into an overlay metal 520 (as described above). Theoverlay metal 520 is now mechanically coupled to a underlying metal 350forming a single green article 500 as shown in FIG. 5.

With reference to the embodiment of FIG. 5 and added reference to FIG.2, the overlay metal 520 of the green article 500 is configured todisplay uniformly porous character whereas the underlying metal 350 isconfigured to display negligible, if any, porosity. Therefore, the greenarticle 500 on the whole is conditioned to provide a tailored gradientof pore character therethrough. Nevertheless, having been consolidatedby CIP alone in the embodiment shown, the green article 500 still lacksthe strength of a sintered metal. Furthermore, pore formers 250 remainin the overlay metal 520 and additional shaping of the green article 500may be desired. Therefore, additional processing of the green article500 may be undertaken as described below.

Continuing with reference to FIGS. 2 and 5, the green article 500 mayoptionally be pre-sintered to increase its tensile strength beforeremoving the pore formers 250. That is, as described below, it may bedesirable to leave the pore formers 250 within the green article 500during any machining to help preserve pore integrity. In the embodimentsshown, the green article 500 may be sintered at between about 500° C.and about 1,000° C. for an hour or more in the presence of an inert gassuch as argon. In this manner sinter bonds may be formed within thegreen article 500 adding to the strength of the compact. In oneembodiment, the homogenizing agent 255 may be dried from the greenarticle 500 at this time if this has not already taken place.

The above described presintering may take place at a temperature highenough to promote development of sinter bonds but at a low enoughtemperature to allow further sintering to take place during laterprocessing. In this manner the trapping of pore formers 250 in the greenarticle 500 is avoided. In light of this concern, the particularparameters of the presintering are largely dependant on the metal 125involved. For example, in an embodiment where titanium powder isprovided as the metal 125, a temperature of 880° C. will not be exceededduring the presintering.

As referenced above, the green article 500 with increased strength andremaining pore formers 250, protecting the integrity of pores, may thenbe subjected to mechanical shaping or green machining. With additionalreference to FIG. 6, in one embodiment, the green article 500, andultimately the porous metal article 600 may be configured to display aroughened surface, or particular texture thereat. For example, the greenarticle 500 may be shaped in this fashion to provide ingrowth surfaceswhere the porous metal article 600 is to be employed as a biologicalimplant.

Continuing with reference to FIGS. 2, 5, and 6, mechanically shaping thegreen article 500 may take place before removal of pore formers 250 andin advance of final sintering as noted above. In this manner, pores atthe surface of the green article 500 are maintained in a relatively openstate by the pore formers 250 themselves during the machining. Thishelps avoid the undesired effect of physically isolating the porosity ofthe porous metal article 600.

In the embodiment shown, the green article 500 is shaped at a lathe 550by the positioning thereof on a lathe support 575. In one embodiment thegreen article 500 is vacuum secured to the lathe support 575 minimizingcontact therewith. The underlying metal 350 is coupled to the lathesupport 575 exposing the overlay metal 520 for shaping by a shapingimplement 525. In the embodiment shown, the surface of the overlay metal520 is affected by the shaping implement 525 such that the outerdiameter of the green article 500 is reduced to a degree. Nevertheless,as indicated above, the presence of pore former 250 in the pores of theoverlay metal 520 protects pore integrity as the shaping implement 525shapes the surface of the green article 500.

Once the green article 500 has been mechanically shaped and finished itmay be removed from the lathe 550 and submerged in an extraction fluidfor removal of pore formers 250 therefrom. In the embodiments describedherein, materials are used in forming the green 500 and porous metal 600articles that lend themselves to a water-based extraction. Use of awater-based extraction helps eliminate environmental, health and safetyconcerns. Nevertheless, other techniques may be employed. For example,other fluids, including gasses and liquids may be used depending on thedissolution properties of the extractable pore-former 250. However, asdescribed herein, the pore formers 250 may be materials such as a metalsalt. The ionic nature of the salt renders it susceptible to extractionby placing the green article 500 in a water bath. That is, the metalsalt is soluble in water.

In order to confirm dissolution of the pore formers 250 into theextraction fluid, conventional monitoring and analysis of the extractionfluid may take place during the extraction. For example, where water isemployed as the extraction fluid, the amount of extractable material inthe water can be measured in ppm by a conductivity meter. Alternatively,reduction of the weight of the transforming green article 500 may bemonitored over the course of the extraction.

As described above, and with continued reference to FIGS. 2, 5, and 6,the pore formers 250 have been removed from the green article 500 toprovide a porous metal article 600. The extraction of the pore formers250 leaves pores in the porous metal article 600 which retain thedimensional properties of the pore formers 250 used to form them. Thishas been achieved in a manner, water-based extraction, that leaves thechemical and mechanical integrity of the articles (500, 600)substantially unaffected. That is, rather than employing hightemperature vaporization, highly reactive solvents or other more severemeasures that might alter the character of the final porous metalarticle 600, the pore formers 250 have been removed by the mereintroduction of water. This is made possible by the particularcombination of metal 125 and pore formers 250 selected. For example, inan embodiment where titanium metal 125 and potassium chloride poreformer 250 are employed, the pore former 250 may be removed from thegreen article 500 by water extraction. This is achieved without damageto the integrity of the metal matrix left behind by either the waterinvolved or the presence of vaporized pore former 250 (i.e. asvaporization has not been utilized).

Continuing with reference to FIGS. 2, 5, and 6, once extraction of thepore formers 250 is completed, final sintering of the now porous metalarticle 600 may proceed to provide added mechanical or tensile strengththereto. This is achieved by way of heating the porous metal article 600to provide sinter bonds throughout. In one embodiment, removal of thehomogenizing agent 255 is also accomplished at this time. Additionally,the final porous metal article 600 may also be subjected to postsintering processes such as hot isostatic pressing (HIP) or otherapplications to provide added density.

The sintering conditions are determined by the properties of the porousmetal article 600 that is being sintered. Times, temperatures, pressuresand atmospheres used in a sintering cycle are selected based on thenature of the material being sintered. In embodiments described hereinthe porous metal article 600 may be placed in a sintering furnace andheated from room temperature at a rate of between about 5° C. to about15° C. per minute. Heating of the porous metal article 600 in thismanner may proceed up to between about 1000° C. and about 1500° C.,preferably between about 1150° C. and about 1400° C. Sintering maycontinue for a period of between about 30 minutes and about 4 hoursunder partial pressure of an inert gas such as argon. The sinteredporous metal article 600 may then be removed from the sintering furnaceand cooled leaving it of increased strength and durability. For example,the porous metal article 600 may display tensile and shear bondstrengths exceeding about 25-50 megapascals (MPa). As described below,the sintered metal article 600 may then be employed in a variety ofapplications such as for use as a biological implant.

While the described process of sintering increases the strength of theporous metal article 600 it also reduces its size proportionally. Assuch, the pore formers 250 as well as the green article 500 areaccordingly configured in an oversized fashion as alluded to earlier.The overall shrinkage of the porous metal article 600 will be dependantthe degree of densification provided thereto by the sintering. This willbe determined by factors such as the dimensions and density of the metal125 and the original mixture 200 as well as its chemistry and the carbonloss or oxygen pick-up during processing. However, as a practicalmatter, embodiments described herein include very consistent andrepeatable techniques. Therefore, once shrinkage is determined for anyparticularly configured and processed porous metal article 600, thepore-formers 250 and green articles 500 may be sized and configuredaccordingly to account for the shrinkage. Thus, precise control over thepore character and sizing of the porous metal article 600 itself may beexercised.

The above-described sintering takes place after the extraction of thepore formers 250 as noted. This reduces the risk of the sinteringleading to contamination of the porous metal article with thepore-formers 250. Thus, in embodiments described herein, neithervaporization, sintering, or any other potentially volatile applicationis employed in such a manner as to risk contamination of the porousmetal article 600 with material of the pore formers 250.

Referring now to FIG. 6, a porous metal article 600 has been formed asdescribed above. In the embodiment shown, the porous metal article 600is to be used as a biological implant in the form of an acetabular shellor cup for a hip joint 610. The article 600 may be provided with abiological coating and implanted into a socket 650 at a pelvis 625 toreceive and secure the head 675 of a femur 680.

As shown, the porous metal article 600 includes a porous portion (i.e.of overlay metal 520) and a non-porous portion (i.e. the underlyingmetal 350). As such, the porous metal article 600 as a whole may beconsidered to be of a particular tailored porosity. That is, from aportion exhibiting porosity to a portion exhibiting none. Further, theporosity of the porous portion of overlay metal 520 is preciselyconfigured and of a uniform pore distribution as provided throughtechniques described above. As such, bone at the socket 650 of thepelvis 625 is provided with a uniquely homogeneous porous matrixsubstrate within which to integrate. By the same token, the head 675 ofthe femur may be secured by the concave and smooth non-porous underlyingmetal 350 of the porous metal article 600. Thus, embodiments describedherein may be employed to provide a single co-formed porous metalarticle 600 displaying the character to provide both physical securityvia a porous matrix (overlay metal 520) as well as flexibility via asmooth non-porous portion (underlying metal 350) of the same material asthe matrix.

With particular reference to the flow-chart of FIG. 7, in addition FIGS.1-6 described above, an embodiment of forming a porous metal article 600of controlled porosity and for use as a biological implant issummarized. As noted at 710 a pore former 250 and homogenizing agent 255are combined to create a coated pore former 100. A metal 125 is blendedwith the coated pore former 250 to form a homogeneous mixture 200thereof as indicated at 720. The homogeneous nature of the mixture 200is maintained by properties of the homogenizing agent 255.

The above described mixture 200 is placed in a mold tool 300 asindicated at 730. The mold tool 300 may be optionally tumbled and tapped(735, 740) before the mixture 200 therein is solidified into a greenarticle 500 as indicated at 750. As noted at 760, the solid greenarticle 500 may be presintered and/or shaped before pore formers 250therein are extracted to form a porous metal article 600 (see 770).Extraction 770 takes place in a manner that preserves and ensures theintegrity of the formed porous metal article 600. In the embodimentshown, the porous metal article 600 may then be finally sintered andemployed as a biological implant as indicated at 780 and 790.

Embodiments of a biological implant may be of a particularly configuredinterconnecting porosity through techniques described above. Forexample, to support bone ingrowth into the porous portion of an article,an interconnecting pore ratio of between about 2:1 and about 5:1 ofmajor pore size to minor pore size may be employed. That is, in an opencell embodiment, any given pore former size employed will result in aporous portion displaying pores relative to the size of the pore former(i.e. major pore size) and porosity relative to the overlap orinterconnectivity of the pores (i.e. minor pore size). The porosity ofthe porous portion in such an embodiment may be between about 60% andabout 85%.

In a preferred embodiment, an interconnecting pore ratio may be betweenabout 2.5:1 and about 4.5:1. Such ratios may be particularlyadvantageous to enhancing bone ingrowth into a porous portion of abiological implant. In one embodiment, a porosity of about 70% isprovided to the porous portion of the article via pore formers of about1,000 microns in diameter. This may result in a minor pore size of about270 microns and thus, an interconnecting pore ratio of about 3.7:1. Inanother embodiment, a porosity of about 70% is provided to the porousportion of the article via pore formers of about 575 microns indiameter. This may result in a minor pore size of about 160 microns andthus, an interconnecting pore ratio of about 3.6:1. Regardless, theinterconnecting pore ratio will be substantially uniform throughout theporous portion of the article.

The embodiments described herein provide techniques of employing solidpore formers to create a porous metal article in a manner that allowsfor a controlled pore character to be displayed by the article withrespect to pore size, morphology and uniformity of distribution. Certainembodiments may even provide for a tailored pore character to extendthroughout the article such that one portion of the article isconfigured of one pore character and level of porosity and anotherportion of a second pore character or level of porosity. Furthermore,porous metal articles may be formed as described in a manner thatremoves solid pore formers without otherwise altering the ultimatephysical or chemical character of the porous metal article itself.

Although exemplary embodiments describe the forming of particular porousmetal articles in the form of layered biological implants, additionalembodiments are possible. For example, filters, flow control devices,and closed cell porous articles may be formed employing techniquesdescribed herein. Furthermore, many changes, modifications, andsubstitutions may be made without departing from the spirit and scope ofthe described embodiments.

1. A porous article comprising a porous portion with an average ratiobetween about 2:1 and about 5:1 of major pore size to minor pore size,having a substantially uniform pore distribution and a porosity of atleast 70 percent, the porous article for use as a biological implant. 2.The porous article of claim 1 wherein the porous portion is a firstportion to support bone ingrowth, the porous article having a secondportion of negligible porosity to avoid bone ingrowth.
 3. The porousarticle of claim 1 wherein the biological implant is an acetabular cup.